EclipseLink Development in Git

This page is for the Git usage portion of the dev process. It does not discuss issues with the build in Git, for more information on that please see: wiki.eclipse.org/EclipseLink/Build/Git .

Brief Overview

Git is a Distributed Version Control System (DVCS), which means there is no single point-of-failure and one can do useful work without a server. The ability to work while disconnected is very useful if a server is down or if the network connection to a server is unreliable, slow or firewalled.

The distributed nature of Git means that the source code is inherently backed-up across all the various 'clones' that may exist 'out there.' In addition, Git supports types of work-flows that are different from those supported by Subversion; these work-flows, while unfamiliar, are quite powerful and can 'overlap' - one developer may prefer a 'golden repository' work-flow while another likes 'trusted-lieutenants'; both can be supported simultaneously by the same repository.

How does Git differ from SVN?

Beyond the obvious distributed vs. central repository concept, one of the great strengths of Git is its on-disk representation of a repository. When you 'checkout' from SVN, you get a working-directory tree of files and folders that represent the 'tip' of that particular SVN branch. If you need to do some operation, say examine the commit history for some file, you must go back to the server to get the metadata for those commits. When you 'clone' a Git repository you (also) get a working-directory tree of files and folders but also the complete commit history for the entire repository. Operations to compare commits or revert the working-directory to a previous state all take place a the speed of the local filesystem without any network round-trips. Further, the history for the repository is stored in an efficient 'packed' representation.

How do patches in Git differ from in SVN?

A normal patch identifies the artifacts that have changed, typically using some directory-path+filename to specify where to apply additions, modifications or deletions. Git patches work mostly the same way, but in addition to path information, each commit (or patch) contains the cryptographic signature (SHA-1) of the parent (or parents) commit that links to it:

Basically, a Git commit (patch) does not live in isolation - it is part of a tree-structure and links to the history of the repository.

How do I get started?

The 'core' Git distribution can be downloaded from the Git download site (http://git-scm.com/download) - there are links for a variety of operating systems (Linux, Mac OS X, Windows).

Committer access through SSH

Step 1: Git Committer Identity

When a commit is made in Git, the commit has metadata identifying two things:

the author (name and email): who created the change, and

the committer (name and email): who committed the change to the repository

(of course for many commits the author IS the committer so only the author is identified)

The Eclipse Foundation uses these fields as part of its IP process - only committers to a project can change source stored on a Foundation's server. However, a committer may make changes on behalf of others - this enables collaboration with parties that have not gone through the Eclipse IP due diligence process. This especially is useful if say the third-party just wanted to contribute a few one-of patches: the administrative overhead of the Eclipse IP due diligence process would likely 'scare-off' most contributions (for more information, please see Handling Git Contributions)

NB. Windows often has difficulty with 'dot-files' in its 'home' directory - you may have to create this file from the command-line. In addition, if your 'home' directory is on a UNC fileshare directory, the msysgit Windows-version of git tools may not be able to read or write it. I solved this issue by redefining two Windows environment variables (HOME and HOMEDRIVE):

Step 2: SSH identity

As mentioned above, most Git servers are set up so that HTTP access is read-only - in order to be able to commit, one must connect to the Eclipse Foundation Git servers over SSH:

The file ending in .pub is the public portion of the key-pair. SFTP the public key, or send an email message asking 'webmaster@eclipse.org' to place it in the appropriate place in your home directory on the Eclipse Foundation's server. The private portion must be moved to your local home directory ~/.ssh/committer.ppk. Now set permissions on your private key:

prompt > chmod 700 ~/.ssh
prompt > chmod 600 ~/.ssh/committer.ppk

Instead of emailing 'webmaster@eclipse.org' you can upload the key yourself:

Creating an SSH identify on Windows

In the 'Key comment' field, replace the entry starting with 'rsa-key ...' with your_eclipse_committer_id@dev.eclipse.org

Save the private key to %HOMEDIR%\.ssh\committer.ppk

Save the text from the box "Public key" into a new file: %HOMEDIR%\.ssh/authorized_keys

The last step is necessary because PUTTYGEN has a custom format for the public portion of the key-pair that will not work when uploaded to the Eclipse Foundation's server. You must save the text from the box 'for pasting' - the key is in the same format as generated by ssh-keygen

Getting through a firewall

In your home ~/.ssh/ directory, you must create a config file

prompt > cd ~/.ssh
prompt > touch config
prompt > chmod 600 config

The documentation for the ~/.ssh/config file can be found here. The particular features to focus upon are:

the ability to specify a particular identity file, and

the ability to specify a command to be run whenever we attempt to connect to a specific host:

Workflow overview

How is merging different between Git and SVN

One fundamental difference between Git and SVN is branching - a branch in SVN is a very expensive artifact while in Git it is extremely lightweight - the additional cost to a local repository clone or a remote server is negligible. This leads to workflows where one creates a new branch for each feature or each refactoring or even each bug. When you are done, it is easy to merge changes from a branch into 'master' (like SVN 'trunk'). Merging can be done either manually or through tools (e.g. Beyond Compare) that can be integrated into the Git merge process.

Other Workflows

Central Repository Workflow
A very common workflow, especially from people transitioning from a centralized system (SVN). Git will not allow you to push if someone has pushed since the last time you fetched, so a centralized model where all developers push to the same server works just fine.

Integration Manager Workflow
Another common workflow is where there is an integration manager — a single person who commits to the 'golden' repository, and then a number of developers who clone from that repository, push to their own independent repositories and ask the integrator to pull in their changes. This is the type of development model you often see with open source or GitHub repositories.

Trusted Lieutenants Workflow
For more massive projects, you can setup your developers similar to the way the Linux kernel is run, where people are in charge of a specific subsystem of the project ('lieutenants') and merge in all changes that have to do with that subsystem. Then another integrator (the 'dictator') can pull changes from only his/her lieutenants and the push to the 'blessed' repository that everyone then clones from again.